The present invention relates to a process of separating pentafluoroethane through extractive distillation process using a particular compound as an extractant (or solvent) from a mixture comprising at least pentafluoroethane (hereinafter, also referred to as xe2x80x9cHFC-125xe2x80x9d) and chloropentafluoroethane (hereinafter, also referred to as xe2x80x9cCFC-115xe2x80x9d), for example a reaction product from a production process for pentafluoroethane through fluorination of tetrachloroethylene.
HFC-125 is a useful compound as an alternative chlorofluorocarbon (or flon) compound free of chlorine and is used as a cooling medium, a blowing agent, an injecting propellant and so on. HFC-125 is conventionally produced through a fluorination process of tetrachloroethylene. In such a production process, dichlorotetrafluoroethane, dichlorotrifluoroethane, hexafluoroethane, CFC-115 and so on are produced as by-products.
Among those by-products, CFC-115 has a boiling point of xe2x88x9238.7xc2x0 C. and HFC-125 which is the desired product has a boiling point of xe2x88x9248.5xc2x0 C. However, a value of the relative volatility between these two compounds is close to 1. Particularly, when a mixture contains HC-125 at a concentration of not less than 95 mole % (that is, the concentration of CFC-115 is not more than 5 mole %), the relative volatility value is about 1.04. Accordingly, when the above mixture is distilled to separate HFC-125 at a high concentration through a conventional distillation treatment, a distillation apparatus having much plate numbers is required, which generally means that the separation using distillation is extremely difficult.
In the present specification, when a solution comprising at least a remarked component A and a remarked component B (a boiling point of component A less than a boiling point of component B) is in a vapor-liquid equilibrium state with the vapor thereof, the term xe2x80x9crelative volatility (xcex1)xe2x80x9d is defined as follows:
xcex1=(YA/XA)/(YB/XB)
wherein, XA is a molar fraction of the lower boiling component A in the liquid phase, XB is a molar fraction of the higher boiling component B in the liquid phase, YA is a molar fraction of the lower boiling component A in the vapor phase which is in equilibrium with its liquid phase, and YB is a molar fraction of the higher boiling component B in such vapor phase.
In order to recover one component from a mixture of a system having a relative volatility value of around 1, an extractive distillation process is adopted. As an example of the extractive distillation process to separate a mixture comprising HFC-125 and CFC-115, U.S. Pat. No. 5,087,329 discloses an extractive distillation process using 1,1,1,2-tetrafluoro-2,2-dichloroethane or 1,1,1-trifluoro-2,2-dichloroethane as an extractant.
In the process disclosed in U.S. Pat. No. 5,087,329, a value of the relative volatility between HFC-125 and CFC-115 is almost 1.2 based on a calculation using figures shown in Example 1 thereof. Thus, when the concentration a 15 of HFC-125 is intended to increase to a ratio of HFC-125/CFC-115=99.7 mole %/0.3 mole % from a mixture having a ratio of HFC-125/CFC-115=7 mole %/93 mole %, a theoretical plate number amounting almost to 40 plates is required in that system according to a theoretical calculation. In this process, a mixture comprising extractant and CFC-115 and HFC-125 is obtained as a bottom product from the extractive distillation step. Thus, it is necessary to sufficiently remove CFC-115 from the bottom product in order to re-use the extractant.
As a result of extensive studies on a process to separate HFC-125 from a mixture comprising HFC-125 and CFC-115 through an extractive distillation process with high efficiency, the present inventors found the matters as follows:
When a mixture comprising at least HFC-125and CFC-115 is subjected to an extractive distillation process, HFC-125 may be efficiently separated from the above mixture by using, for example, as an extractant (or a solvent), at least one compound (thus, a single compound or a mixture of two or more compounds) selected from the group of hydrofluorocarbon compounds having two carbon atoms.
Accordingly, the present invention provides a process of separating HFC-125 from a mixture comprising at least HFC-125 and CFC-115 through an extractive distillation process, characterized in that compound (thus, a single compound or a mixture of two or more compounds) selected from the group of hydrofluorocarbon compounds having two carbon atoms at least one compound selected from the group of hydrofluorocarbon compounds having two carbon atoms is used as an extractant to obtain HFC-125 in which a concentration of CFC-115is relatively reduced, and preferably highly concentrated HFC-125 which is free of CFC-115.
That is, the present invention provides a process of producing pentafluoroethane in which a mixture comprising at least pentafluoroethane and chloropentafluoroethane as a main component thereof is subjected to an extractive distillation to obtain another mixture which contains pentafluoroethane as a main component thereof and does not substantially contain chloropentafluoroethane,
characterized in that at least one compound selected from the group of hydrofluorocarbon compounds having two carbon atoms is used as an extractant to obtain a mixture comprising pentafluoroethane and the extractant as a main component thereof as a bottom product of the extractive distillation step.
In the present specification, the term xe2x80x9cmain componentxe2x80x9d means that the content of the other components is relatively smaller than the content of the main component in the mixture. Concretely, it is sufficient that an amount of the main component is not less than 50%, more concretely not less than 60%, for example not less than 80% (these amount may be on a molar or weight basis). Additionally, in the present specification, the phrase xe2x80x9cpentafluoroethane substantially free of chloropentafluoroethanexe2x80x9d means that the finally obtainable product is a mixture including pentafluoroethane as its main component, wherein for example, such a mixture may have a concentration of pentafluoroethane not less than 90 weight %, preferably not less than 99.9 weight %, and more preferably not less than 99.99 weight %.
According to the present process when at least one compound selected from the group of hydrofluorocarbon compounds having two carbon atoms is used as an extractant, a mixture may be obtained as a bottom product, which contains pentafluoroethane and the extractant as the main component thereof, preferably a mixture containing chloropentafluoroethane at a concentration not more than 0.1 weight %, and more preferably not more than 0.01 weight %. In this case, the composition and the amount of the distillate product from the extractive distillation is not particularly limited, provided that a ratio of chloropentafluoroethane to pentafluoroethane in the bottom product is reduced by somewhat an extent than an original ratio thereof, preferably the ratio is reduced to not more than 1/10 of the original ratio, and more preferably reduced to not more than 1/100 of the original ratio. In addition, the distillate product may contain, as a main component thereof, chloropentafluoroethane or pentafluoroethane or a mixture of chloropentafluoroethane and pentafluoroethane.